It remains largely unknown how energy metabolism coordinates hematopoietic stem cell (HSC) maintenance and lineage differentiation. The hypoxic microenvironment in stem cell niches limits mitochondrial aerobic metabolism (respiration/oxidative phosphorylation) in HSCs, which preserves this essential cell reservoir from oxidative damage by attenuating the production of reactive oxygen species (ROS), a byproduct of mitochondrial respiration. However, cell intrinsic mechanisms regulating HSC metabolic activities are poorly defined. Furthermore, how energy metabolism orchestrates HSC differentiation in concert with other regulatory networks has not been characterized. Lack of such knowledge impedes the understanding of the pathogenesis of blood diseases associated with mitochondrial dysfunction and metabolic conditions. PTPMT1, an evolutionarily conserved PTEN-like phosphatidylinositol phosphate (PIP) phosphatase, is localized to the mitochondrial inner membrane where ion channels and transporters, important for mitochondrial ion homeostasis and thus metabolism, reside. Our preliminary studies have shown that targeted disruption of PTPMT1 in mice results in post-implantation embryonic lethality. Deletion of PTPMT1 from adult bone marrow (BM) cells of inducible knockout (PTPMT1fl/fl/Mx1-Cre+) mice impairs myeloid and lymphoid cell development. Moreover, postnatal hematopoiesis in hematopoietic cell-specific knockout (PTPMT1fl/fl/Vav1-Cre+) mice is completely blocked. These mice succumb to pancytopenia within 3-6 days of birth. Strikingly, HSCs in the BM are increased by ~30-fold and ~10-fold in PTPMT1fl/fl/Mx1-Cre+ mice and PTPMT1fl/fl/Vav1-Cre+ neonates, respectively. Preliminary mechanistic studies show that cellular respiration of PTPMT1-depleted cells is decreased while glycolysis is enhanced. The objective of this application is to extend these studies to further determine the role and signaling mechanism of mitochondrial phosphatase PTPMT1 in HSCs. We hypothesize that PTPMT1 coordinates HSC homeostasis and lineage commitment by modulating mitochondrial metabolism. We plan to test our hypothesis and accomplish the objective of this application by pursuing the following two aims. (i) To define the role of PTPMT1 in hematopoiesis. (ii) To determine the mechanisms by which PTPMT1 modulates HSC function. The proposed work is innovative, because it explores the significance of finely controlled mitochondrial metabolism for HSC maintenance and lineage differentiation. The combination of the work proposed is collectively expected to yield novel insights into the bioenergetic regulation of hematopoietic cell development. In addition, the information gathered will lead to a better understanding of the pathophysiology of blood disorders resulting from mitochondrial dysfunction and metabolic conditions.